honestly, the answer to this is ‘it depends’, ‘what are you doing?’, ‘do you appreciate the limitations?’
none of these platforms are laptop replacements, they are all ‘low powered’ devices , and all (except the rPI) are dedicated music devices… each then has a different take on the problem space.
Axoloti - least powerful, but has a proper real time OS, so all the power is going to DSP, if you write your patches/objects correctly , it will perform to its ‘breaking point’ pretty seamlessly/reliably. it also has a specific ‘musical’ patching environment, which frankly i think is better than PD. BUT its not as common, so is obviously more specialised/less stuff to 'borrow’
yes, the int maths is a pain, and is partly only necessary due to a lack of an FPU … BUT its the same processor as used by MI in Clouds/Elements which does all its computations in float, so you can use floats, if you are willing to spend the cpu cycles!
Organelle - this is about a proper finished form factor, so users can share patches (as you’ll see on patchstorage), its a musical instrument first and foremost - its powerful enough to do great things, and PD is popular. but like all these platforms, you have to work within constraints .
the OS used (arch Linux) has been nicely trimmed back to not having lots of ‘stuff’ you don’t need running.
rPI3 - sure more powerful than the Organelle, but does not have a musical focus, efforts of devs/users are distributed.
you can make it a musical device, but you need to spend a lot of time doing this, adding hardware (an audio interface is a necessity!) , slimming down a distro, writing a PD (or equivalent) launcher. you will find countless 1 off projects doing this… but even for a developer, its a time consuming task…
is it cheaper? well… look at GR1, its a PI + knobs+ stm controller + display = $800, so no, once someone does all the legwork… its not cheap…
if you do it DIY, sure , expect to add about $100-150 to the PI price, for enclosure, audio interface… then another $100-150 for control (midi) + an lcd display… its surprising how it adds up.
Bela - is somewhere between Axoloti and Organelle, its using a real time OS, so like Axoloti can guarantee latency, but has 1ghz processor, and is Linux based, but the software support is no where near Organelle/Axoloti (though it will run libPD) , so leans towards the DIY/rPI.
but arguably for building a diy hardware instrument, which needs more grunt than Axoloti can offer… its a great offering. (but its not cheap)
as for software/filters… well Organelle is no more tied to PD than a rPI or Bela…
you can run the same software as these.
then bare in mind the DAC used is going to play a part, seems nice on the Organelle, the Bela is ok, and the rPI needs to be supplemented with something else, the built-in is dreadful, so depends what you replace it with… and what latency this introduces.
p.s. i don’t think usb soundcards on the rPI are a good idea, its USB is not fantastic anyway… so id be concerned about latency/jitter - however, Ive both a HiFi Berry and a Blokas board, the latter I think is pretty good, and has ‘made’ the PI for me… but it does steal a lot of gpio!
in short, Ive all the above, and I don’t think any is better or worst than the others …
for hardware hacking, fx or quick patching , I still love and use the Axoloti, everyday.
for playing/enjoying/experimenting, I use the Organelle daily.
Blokas have made my rPI more attractive but along with Bela, they are sitting there in the wings waiting for development… the irony being, that development is taking place on the Organelle, and then will be pushed to the rPI/Bela. I guess I kind of see the rPI/Bela as headless DSP boxes, so basically, I can do the musical dev on the Organelle, where I can easily ‘play’ with its form, play with the fx/sound generators, and then when happy with them, farm them out to a faceless black box